Pointer Arithmetic B T W. *(array_ptr++) 1 vs (*array_ptr)++

Transcription

1 Pointer Arithmetic When you add to or subtract from a pointer, the amount by which you do that is multiplied by the size of the type the pointer points to. In the case of our three increments, each 1 that you added was multiplied by sizeof(int). int array[] = { 45, 67, 89 }; int *array_ptr = array; printf(" first element: %i\n", *(array_ptr++)); 1 printf("second element: %i\n", *(array_ptr++)); printf(" third element: %i\n", *array_ptr); Output: first element: 45 second element: 67 third element: 89 NOTE 1: 1==4 (programmer humor?!) *(array_ptr++) == *array_ptr++ B T W *(array_ptr++) 1 vs (*array_ptr)++ find the value at that address, output, then add 1 to the address VS Find the value at the address, output, then add one to the value at that address

2 Pointer Arithmetic (cont) Expression Assuming p is a pointer to a and the size of *p is Value added to the pointer p+1 char 1 1 p+1 short 2 2 p+1 int 4 4 p+1 double 8 8 p+2 char 1 2 p+2 short 2 4 p+2 int 4 8 p+2 double

3 Pointer Arithmetic (again) pointer (+ or -) integer Only for pointers that are pointing at an element of an array Also works with malloc Watch for bounds (begin and end) Ok to go one beyond the array but not a valid dereference pointer#1 pointer#2 Only allowed when both point to elements of the same array and p1 index < p2 index Measured in array elements not bytes If p1 array[i] and p2 array[j] then p2-p1 == j - i 98

4 Pointer Indexing int array[] = { 45, 67, 89 }; printf("%i\n", array[0]); // output is 45 // array and array[0] point to same thing The subscript operator (the [] in array[0]) has nothing to do with arrays. In most contexts, arrays decay to pointers. This is one of them: That's a pointer you passed to that operator, not an array. int array[] = { 45, 67, 89 }; int *array_ptr = &array[1]; printf("%i\n", array_ptr[1]); //output is 89 (whoooooooaaaahhhhtttt??!!) array points to the first element of the array; array[1] == *(array + 1) array_ptr is set to &array[1], so it points to the second element of the array. So array_ptr[1] is equivalent to array[2] 99

5 NULL vs 0 vs \0 NULL is a macro defined in several standard headers 0 is an integer constant '\0' is a character constant, and nul is the name of the character constant. All of these are *not* interchangeable NULL is to be used for pointers only since it may be defined as ((void *) 0), this would cause problems with anything but pointers. 0 can be used anywhere, it is the generic symbol for each type's zero value and the compiler will sort things out. '\0' should be used only in a character context. nul is not defined in C or C++, it shouldn't be used unless you define it yourself in a suitable manner, like: #define nul '\0' 100

6 NULL pointer and VOID 0 (an integer value) is convertible to a null pointer value if assigned to a pointer type VOID no value at all literally means nothing So it is type-less (no type defined) so can hold any type of pointer We cannot perform arithmetic on void pointers (no type defined) Cannot dereference (can t say, get the value at that address no type defined) NULL is defined as 0 cast to a void * pointer #define NULL (void *) 0; FYI: However, NULL and zero are not the same as no returned value at all, which is what is meant by a void return value (see your first C program examples) Is there any difference between the following two statements? char *p=0; char *t=null; NO difference. NULL is #defined as 0 in the 'stdio.h' file. Thus, both p and t are NULL pointers. Is this a correct way for NULL pointer assignment? int i=0; char *q=(char*)i; // char * cannot point to an int type even for a moment in time NO. Correct char *q=0 (or) char *q=(char*)0 Is the NULL pointer same as an uninitialized pointer? NO 101

7 R and L values L-value = something that can appear on the left side of an equal sign A place i.e. memory location for a value to be stored R-value is something that can appear on the right side of an equal sign A value Example: a = b+25 vs b+25 = a Example: int a[30]; a[b+10]=0; Example: int a, *pi; pi = &a; *pi = 20; 102

8 R and L values (cont) Given: char ch = a ; char *cp = &ch; NOTE: the? is the location that follows ch cp ch a? Problem Expression R-value L-value 1 ch yes yes 2 &ch yes illegal 3 cp yes yes 4 &cp yes illegal 5 *cp yes yes 6 *c+1 yes illegal 7 *(c+1) yes yes 8 ++cp yes illegal 9 cp++ yes illegal 10 *++cp yes yes 11 *cp++ yes yes 12 ++*cp yes illegal 13 (*cp)++ yes illegal 14 ++*++cp yes illegal 15 ++*cp++ yes illegal 103

9 An Array of Character Pointers #include<stdio.h> int main() { // Declaring/Initializing 3 characters pointers char *ptr1 = "Himanshu"; char *ptr2 = "Arora"; char *ptr3 = "TheGeekStuff"; //Declaring an array of 3 char pointers char* arr[3]; // Initializing the array with values arr[0] = ptr1; arr[1] = ptr2; arr[2] = ptr3; //Printing the values stored in array printf("\n [%s]\n", arr[0]); printf("\n [%s]\n", arr[1]); printf("\n [%s]\n", arr[2]); return 0; } 104

10 Pointers to Arrays <data type> (*<name of ptr>)[<an integer>] Declares a pointer ptr to an array of 5 integers. int(*ptr)[5]; #include<stdio.h> int main(void) { char arr[3]; char (*ptr)[3]; arr[0] = 'a'; arr[1] = 'b'; arr[2] = 'c'; ptr = &arr; return 0; } Declares and initializes an array arr and then declares a pointer ptr to an array of 3 characters. Then initializes ptr with the address of array arr. int *arr[8]; int (*arr)[8]; // An array of int pointers. // A pointer to an array of integers 105